Process for separating hydrocarbons



All@ 29, 1944. FQ E. FREY PROCESS FOR SEPARATING-H/YDROCRBONS Filed June 4, 1942` 2 sheets-sheet 1 Aug.A 29, 1 944. F. E. FREY PROCESS FOR SEPARATING HYDROCARBONS v 'iled June 4. 1942 2 Sheets-Sheet 12A N wn-loo Nouavalxa 7 STORAGE FREDERICK E. FREY Patented Aug-29, 1944v i UNITED STATES PATENT OFFICE raocsss Foa SEPARATING HYDRO- cABnoNs Frederick E. Frey, Bartlesville, om., assignmto Phillips Petroleum Company, a corporation of Delaware gaseous hydrocarbon Application June 4, 1942, Serial No. 445,786

Claims.

tures, produced by thermal cracking of low-mo- -lecular-weight hydrocarbons or in other ways,

that contain the valuable hydrocarbon butadiene and various other saturated and unsaturated four-carbon-atom hydrocarbons which have boiling points quite close to that of butadiene; Separation and puriication of butadiene from gas mixtures is ordinarily necessary in order to utilize the butadiene, as for the production of rubbery polymers or the like. As is known in the art, hydrocarbons containing an acetylenic vlinkage are particularly undesirable impurities in butadiene that is to be used for chemical synthesis.

Examples of acetylenic compounds that have four carbon atoms per molecule and that boil relatively near to butadieneare ethyl acetylene and vinyl acetylene. The production of butadiene vfree from such undesirable impurities, notably acetylene, is quite diillcult and costly byl vinyl known methods oi' fractional distillation because of such factors as the relatively small boiling point dierences that are involved, the tendency for azeotropic, constant-boiling mixtures of hydrocarbons of diierent types to be formed and to interfere with the fractionation, and the large energy requirements of `complicated distillation procedures.

It is an object of my invention to provide a process whereby a gaseous mixture oi.' light hydrocarbons containing butadiene and acetylenic hydrocarbons of four carbon atoms per molecule may be separated into fractions rich in butadiene and in acetylenes, respectively, by treating the ts mixture with suitable solven It is a further object of my invention to provide a process whereby substantially pure butadiene may be recovered from a gaseous mixture of light hydrocarbons containing butadiene and acetylenic hydrocarbons of four carbon jatoms A per molecule.

preferably in countercurrent fashion, the gweousA hydrocarbon mixture with a selective solvent at will be A apparent ture into a fraction rich in vinyl and ethyl acetylenes and into a :fraction rich in butadiene, and

a suitable temperature and pressure. I have found that, under suitable conditions. the fourcarbon-atom acetylenes tend` to selectively dis- -solve in, and to be carried away by, the solvent,

whereas the remaining gaseous phase is enriched in butadiene and is impoverished in acetylenic constituents. Y

I may apply my process in treating' hydrocarbon mixtures .that contain substantial amounts of hydrocarbons of molecular weight lower than that of four-carbon hydrocarbons. My process is likewise applicable in treating hydrocarbon mixtures containing minor amounts of materials heavier than four-carbon hydrocarbons.

An advantage of my process resides in the fact that it is particularly well adapted to treating, at low cost, large volumes oi hydrocarbons that contain only small amounts of the objectionable acetylenic compounds. In such cases the acetyienic compounds may be absorbed by quite limited amounts of solvent, vand economies in the size of equipment and heat requirements are realized as compared with known methods of fractional distillation.

A further advantage of my process over known methods of fractional distillation isz-found in instances in which butene2 is a component of the hydrocarbon mixture to be treated. In such cases, by application of my process in one form, YI can, if desired, separate the hydrocarbon mixthis latter fraction will also contain substantially all'oi' the butene-Z that was in the original mixture. When a similar separation is attempted by conventional .fractionation it is -i'ound that the tendency of butene-2 and vinyl acetylene to form a minimum-boiling constant-boiling mixture causes dimculty and, in consequence, in order to make sure the butadiene-rich lfraction is freed of acetylene-s, it becomes necessary to regulate the fractionation so that only a part, if any, of the butene-2 is incorporated intothe butadienerich fraction, while producing an acetylene-rich fraction that contains most, it not all, of the butene-2.

In some cases, -when treating hydrocarbon mixtures according to my process, I have found it desirable lto subject the butadiene-rich .gas

fraction that isproduced to further treatment from the gaseous mixture. This secondary solvent should b'e of such a 'nature that the primary solvent may be separated from it in some fashion, as by distillation. It will normally be desirable to employ as a secondary solvent a material with a boiling point, approximately 100 C. higher than the boiling .point of the primary solvent.

In accordance with my invention, I take a stream consisting essentially of C4 hydrocarbons including the butadiene and thev alkyl and alkenyl acetylenes to be separated 'and usually substantial amounts oi butanes and/or butenes, and occaslonalLy also containing some C hydrocarbons of the same general type. This stream may be already .available or may be prepared from available hydrocarbon streams in any desired manner. 'I'his stream is selectively extracted in the gaseous phase with a primary solvent which is selective i'or the acetylenes in preference to the butadiene under conditions including temperature, pressure, etc. causing the acetylenes tobe selectively dissolved in the solvent.

If desired, and usually it will be preferable, the butadiene-rich residual gas fraction may be contacted with a secondary selective solvent for the primary solvent and essentially a non-solvent for butadiene, in order to recover any traces of the primary solvent from the residual gas. This .secondary solvent should be one which is readily separable from theppri'mary solvent-in anyr suit- I able manner for example-by fractional distillation, and to this end should boil at least 100 C.- above the primary solvent.

In one embodiment, my invention may involve taking a stream containing the C4 hydrocarbons mentioned above, namely the butadiene and the alkenyl and alkyl acetylenes and butanes and/or butenes, with or without corresponding C5 hydrocarbons. and containing in addition C: hydrocarbons, and treating this stream preliminarily with a selective solvent such that and under conditionsssuch that the butadiene and the C4 acetylenes are dissolved therein in preference to the butanes, butenes` and C: hydrocarbons, following which the dissolved butadiene and acetylenes mixture may be treated in accordance with the foregoing to effect resolution into a butadienerich fraction on the one hand and an acetylenerich fraction on the other hand. Some Cs hydrocarbons includingany Cs dloleilns'and Cs acetylenes may be dissolved in the, solvent along with the butadiene and the C4 acetylenes. This prehminary selective lsolvent extraction may -be brought about vby suitablefadjustment of the extraction conditions infa manner which willy be obvious to those skilled in the art in the light of this disclosure.

In the yaccompanying drawings, Figure 1 and Figure 2 are schematic now-,diagrams of speciiic embodiments that exemplify the practice of this invention. f j

In Figure 1, the gaseous hydrocarbon mixture to be treated is assumed to be that produced by low pressure cracking oi propane or o! a mixture of light parafnic hydrocarbons with an average molecular Weightv about that of propane. The cracked gas so produced is assumed to have been passed through a tar trap and subjected to one after compression, is introduced by line I2 into oil absorption tower i, down which ows mineral seal oil in countercurrent contact with the cracked gas. The least soluble components of the cracked gas issue I3. The rich oil, containing relatively soluble components of the cracked gas, is passed from the bottom of tower l through line I4 into `ilash tank i6, where the most volatile hydrocarbons dissolved in the rich oil are allowed to flash oi! and where some reduction in temperature occurs. The light hydrocarbons so ashed from the oil are withdrawn through line I1, and the oil proceeds by line I8 and pump I9 into column 2, where further stripping of/ the oil occurs. Column 2 is operated to produce a mixture oi fourcarbon and lighter hydrocarbons issuing overhead from the column through line 20 into condenser 2l, where they are condensed; the condensate passes to accumulator 9. A portion o! the oondensate from accumulator s is returned as-reilux to column 2, as through line 22k and pump. The absorption oil reaching the kettle of column 2 passes 'through line 24 and pump 2l into column 6, where all remaining absorbed materials, conto storage tank 29. Oil from storage tank 2l is recycled by means of line I0 and pump 3| to column l.

Condensate from accumulator 9, containing four-carbon, three-carbon, and small amounts of lighter hydrocarbons. is passed by line I2 and pump 33 into column 3. In column l a depropanizing operation is carried out so that overhead product reaching accumulator I0 through line 34 and condenser 35 contains mainly threecarbon and any lighter hydrocarbons present in the feed to column 3. Reilux Ais provided'ior A column 3 by pumping a portion of the condensate in accumulator i0 back into the top of the col-v umn by means of pump and line.31. It may be noted `that in some cases only a very rough depropanization may, be required, since rather considerable amounts of lighter-than-four-carbon materials vmay be Atolerated. in the subsequent extraction step.

The four-carbon fraction obtained as kettle product from column 3 is conducted through line 38 and pump 39 into absorption tower 4, where countercurrent contacting of the vapors with 'a primary selective solvent 'effects absorption o!l four-carbon acetylenes, so that butadiene-richl vapor passes overhead whereas vinyl andV ethyl acetylenes are removed in the selective solvent. Selective solvent containing dissolved hydrocarbons proceeds from the kettle of absorption tower 4 through line 39 into' stripping column 1, where y the dissolved hydrocarbons are' stripped from the solvent; the vapors so produced are removed through line 40 as a fraction rich in acetylenic constituents. Stripped solvent vpasses from the kettle of column 1 to cooler 4I, from whence it is recycled through pump 42 and line 4l to abor two stages oi compression. The cracked gas,

sorption tower 4.

The butadiene-rich four-carbon fraction produced as overhead from absorption tower 4 contains a certain amount of primary solvent vapor; in order to prevent loss of solvent, as well as oon'- tamination of the hydrocarbon stream, this iraction in gaseous form is conducted by line 42 into extraction tower 5, where a countercurrent coni'fting with a secondary selective solvent is csrfrom the top of the tower through line dissolved primary solvent and is passed through line 45 into stripping column 8, whereprimary .solvent is distilled from the secondary solvent.

If desired, the fractionation may be conducted at atmospheric or subatmospheric pressure. 'I'he primary solvent vapor produced as overhead in column 8 passes by line 41 to condenser 48 and thence to accumulator Il. condensate is recy.- cled from accumulator I I to absorption tower 4 as by means of line 49 and pump 50 and through line 5|, pump 4 2, and line 43. It desired, provision may be made for returning apart of the condensate from condenser 48 as reiiux'to the top of stripping column 8. The stripped secondary solvent from the kettle of stripping column 8 passes tol cooler 52, and thence by line 53 and pump 54 is recycled to extraction tower 5.

It is to be understood that in each of the towers or columns there are provided suitable means for ensuring countercurrent .gas-liquid contacting,

` such as trays with bubble caps, packing. or the like.

The amount of -primary selective solvent required per unit quantity of hydrocarbon mixture treated is, as will be recognized by those skilled in the art, dependent upon such factors as the particular solvent utilized, the percentage of acetylenic compounds present in the hydrocarbon mixture, the extent to which these are to be removed, the temperature and the pressure at which the process is operated, and the efiiciency of the equipment employed to contact solvent and gaseous hydrocarbons.

in cases in which the acetylenic compounds to be removed are present in relatively minor quantities, that is, under 5 to 10 per cent, then with furfural as the primary selective solvent, preferred operating conditions are: (1) solvent rate in the range oi 0.1 to 3 moles solvent per mole oi hydrocarbons treated; (2) pressure in the extraction column in the range of about 50 to 100 pounds per square inch; and (3) temperaturel within the column so adjusted as to maintain the desired pressure. In the stripping of dissolved acetylenes from the rich solvent, somewhat lower pressure and higher temperature than those prevailing in the extraction column are desirable. In case a primary solvent is chosen from the group including furfural, nitrobenzenc, dichlorodiethyl ether, dimethyl formamide, methyl levulinate, glycol diacetate, diethyl acetamide, pyridine, phenol, phenol plus water (containing l20% of water), and benzyl alcohol, both the extraction and thestripping may preferably be carried out at somewhat lower pressure orv at higher temperature than in the case of furfural.

In connection with the stripping of rich solvent, it is to be noted that the hydrocarbons evolved may be rich in vinyl acetylene. This compound exhibits a strong polymerizing tendency when in high concentration, and under some conditions diiiiculty soluble polymers may be formed. In order to avoid diillculties from this source. provision may\be made for passing a stream of relatively insoluble, inert, non-condensible, ldiluent gas, such as nitrogen, methane, or the like,l

into the stripping column 1 via line 40 `at a point below thepoint of introduction of rich solvent. In this manner the rate of polymerization .of

vinyl-acetylene may be greatly reduced, and thus a valuable material is recovered in better yield and with decreased danger of resinous deposits being formed in the equipment. `This feature may be employed wherever a stream comprising alkenyl acetylene is stripped. i. e., in the strip- `ping of the primary or of the preliminary rich solvent.

, I may choose a secondary selective-solvent from a large group of quite high boiling organic.

materials that are somewhat polar in nature and that preferably exhibit reasonably good thermal `stability. Some individual members of this group are: diphenyl amine, ditolyl amine, esters of phthalic acid, nitrogen bases such as qulnollne, and ethyl sebacate.

H- Quite a wide range of operating conditions and Ygas-to-'secondary solvent ratios may be used satisfactorily Iin the recovery of primary solvent vapor. In most cases satisfactory operation will be had at pressures of to 100 pounds per square inch and at temperatures slightly above the dew point temperature for thegaseous mixture being treated. Only limited volumes of secondary solvent, of the order of 0.01 to 0.2 moleper mole of gas to be treated, are required.

In Figure 2, the initial steps of oil absorption, stripping, and the like are substantially the same as in the process of Figure 1,'and are omitted for sake of simplicity since the description of them made hereinbefore in connection with Figure l is believed to be sumcient. Subsequent steps are carried out as follows:

Condensate from accumulator tl, containing threeand four-carbon hydrocarbons and small amounts of lighter hydrocarbons, is passed by line 32 and pump 33 through vaporizer 6d into extraction column M3. A stream of a selective solvent,'such as furfural, is introduced into column |03 through line 55 in such fashion as to bring about countercurrent extraction of these hydrocarbons. Column 03 is operated under lsuch conditions of` temperature, pressure, and

volumes of solvent and hydrocarbon feed as to effect extraction of butadiene and four-carbon acetylenes by the solvent. Undissolved hydrocarbons pass overhead from column "is via line 66 and condenser 61 to accumulator NB. Reflux liquid may be returned to column 903 from accumulator |08 by means of line 60 and pump 8l. The hydrocarbons produced overhead from column |03 may be withdrawn fromthe system by line and may comprise three-carbon and lighter hydrocarbons together with butanes and butenes. Solvent containing dissolved butadiene and four-carbon acetylenes together with minor amounts' of less soluble hydrocarbons and possibly some rive-carbon hydrocarbons is passed from column |03 through line 'l0 and pump Ii into stripping column |00. By application of heat vand/or by reduction of pressure the dissolved From storage tank 15 solvent may be recycled Athrough pump 10 and line B5 to extraction column |03.

The hydrocarbons passing overhead from stripping column Il! through line 'I2 into column v |04 are there subjected to countercurrent vaporliquid extraction under such conditions of temperature,pressure, and volumes of solvent and hydrocarbon feed that a separation of butadiene from the four-carbon acetylenes is eilected. Selective solvent is introduced by line 89 into column |04. A butadiene-rich fraction is taken om overhead from column |04 by line 11 into condenser and thence to accumulator |05. If desired, liquid from accumulator |05 may be returned as reflux liquid to the top of column |04 by means of pump 19 and line 80. The butadiene concentrate produced is withdrawn from accumulator |05 through line 8| to storage or subsequent processing steps, not shown. Solvent containing dissolved hydrocarbons, largely fourcarbon acetylenes, is passed from the kettle of column |04 through pump 82 and line 03 into solvent stripping column |01. In column |01 the solvent is freed of dissolved hydrocarbons, which pass overhead and are removed from the system through line 8l. Stripped solvent is passed from the kettle of column |01 through cooler 85 to storage tank 86. Solvent is recycled to column |04 from tank 86 via line 81, pump 08, and line 89.

The separation eiected in extraction column |03 is made by taking advantage of the fact that both butadiene and four-carbon acetylenes are more soluble in certain selective solvents than are butanes, butenes, and three-carbon hydrocarbons. Because the butadiene and the fourcarbon acetylenes are more soluble in the solvent, they are withdrawn from the kettle ofcolumn |03 as solute in the solvent, while the abovenamed less soluble hydrocarbons pass overhead from the column in vaporous form. A's will be recognized by those skilled in the art, the exact conditions of temperature, pressure, internal reiiux in the column, and relative volume of solvent' to be employed will depend upon the particular solvent, the exact composition oi. the feed stock, and the completeness of separation that is to be realized. Whenfurfural is used as the selective solvent, the operating pressure preferably lies in the range of two to ten atmos` pheres with temperatures within the column so y adjusted as to maintain the desired pressure.` In cases in which the butadiene plus four-carbon acetylenes amount to only a few per cent of the hydrocarbon feed, the preferred solvent rates lie in the range of 2 to 20 moles solvent per mole of hydrocarbon feed. For any particular application, optimum conditions may be readily determined by trial. In the operation of stripping the rich solvent of dissolved butadiene and acetylenes, somewhat lower pressure and higher temperature than those prevailing in the extrction column are desirable.

Alternatively to furfural, suitable selective solvents for Aseparation of butadiene plus fourcarbon acetylenes from butanes, butenes, and lighter hydrocarbons may be chosen from the group of primary solvents indicated hereinbefore.

'I'he separation effected in extraction column |00 is made by taking advantage of my discovery that four-carbon-atom acetylenes tend to be dissolved selectively by certain solvents when a gaseous mixture comprising -said acetylenes and butadiene is contacted with solvent. The same considerations as to operating conditions and choice of solvent apply to the separation effected by extraction column |04 as vhave been indicated Y hereinbefore forabsorption tower l of Figure l.

In order to prevent or minimize polymerization of the vinyl acetylene, a stream of diluent gas of the type described above may be introduced via line ||0 into primary stripping column |01.

If desired a similar stream may be introduced. 'Il

via linef||2 into preliminary stripping column Because the invention may be practiced otherwise than as specifically shown herein, and be-Y cause many modifications and variations offit will be obvious to those skilled in the art, it should not be restricted except' as specifically indicated in the appended claims.

solved in gaseous phase and essentially'free from four carbon atom acetylene, and separating the resulting solvent enriched in four carbon atom acetylene from the resulting butadiene-enriched gaseous phase essentially free from four carbon atom acetylene.

2. The process of eil'ecting resolution of a mixture of butadiene and ethyl acetylene into a fraction rich in butadiene and essentially free from ethyl acetylene and a fraction rich in ethyl acetylene which comprises extracting said mixture in gaseous form with an organic liquid selective solvent for the ethyl acetylene and thereby effecting preferential solution of said ethyl acetylene in said solvent while causing the butadiene to pass through undissolved in gaseous phase and essentially free from ethyl acetylene, and separating the resulting solvent enriched in ethyl acetylene from the resulting butadiene-enriched gaseous phase essentially free from ethyl acetylene.

3. The process of effecting resolution of a mixture of butadiene and at least one four carbon atom acetylene into a fraction rich in butadiene and essentially free from four carbon atom acetylene and a fraction rich in the four carbon atom acetylene content of said mixture which comprises extracting said mixture Vin gaseous form with furiural and thereby effecting preferential solutionlof said four carbon atom acetylene in said furfural while causing the butadiene to pass through undissolved in gaseous phase and essentially free from four carbon atom acetylene, and separating the resulting furfural enriched in four carbon atom acetylene from the resu1t.

in gaseous phase, separating the rich solvent from the'residual undissolved gas, stripping said dissolved four carbon atom acetylene and butadiene content from said rich solvent, contacting said stripped four carbon atom acetylene and buta- ,ssepso diene in the gsseous phase with sn organic liquid selective soinnt and thereby eilecting` preferential dissolution therein oi seid four carbon stom acetylene content while leaving the butadiene undlssolved und essentially free from four csr- 'bon atom acetylene: and in the gaseous phase,

and separating the resulting four carbon atom acetylene-enriched solvent from the resulting butadiene-enriched gaseous phase essentially free from four carbon atom aoetylenes.

5. The process of claim 4 wherein the said selective solvent used in both of said contacting steps is furfural. v

FREDERICK E. FREY. 

